Application Information: continued
VIN(LOW). Otherwise, two voltage dividers have to be used
to program OV and UV separately.
V
OUT
Ier
R1
R2
R3
R1
FB
VIN
-
Ri
COMP
+
+
VUV
VOV
-
1.27
R2
Figure 11. OV/UV Monitor Divider.
Figure 10. The design of feedback voltage divider has to consider the
error amplifier input current.
Design voltage dividers for OV and UV detection
In Fig.11, the voltage divider uses three resistors in series
to set OV and UV threshold seen from the input voltage.
The values of the resistors can be calculated from the fol-
lowing three equations, where the third equation is
derived from OV hysteresis requirement.
R2 + R3
R2 + R3 + R1
VIN(LOW)
×
= 1V
(A)
(
)
R3
VIN(HIGH)
×
= 2V
(B)
(C)
(
)
R2 + R3 + R1
12.5µA × (R1 + R2) = VHYST
where
VLINE(LOW), VLINE(HIGH) = input voltage OV and UV threshold
VHYST = OV hysteresis seen at VIN
It is self-evident from equation A and B that to use this
design, VIN(HIGH) has to be two times greater than
11
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
